Polycrystalline diamond compacts (“PDC”) have been used in industrial applications, including rock drilling applications and metal machining applications. Such compacts have demonstrated advantages over some other types of cutting elements, such as better wear resistance and impact resistance. The PDC can be formed by sintering individual diamond particles together under the high pressure and high temperature (“HPHT”) conditions referred to as the “diamond stable region,” which is typically above forty kilobars and between 1,200 degrees Celsius and 2,000 degrees Celsius, in the presence of a catalyst/solvent which promotes diamond-diamond bonding. Some examples of catalyst/solvents for sintered diamond compacts are cobalt, nickel, iron, and other Group VIII metals. PDCs usually have a diamond content greater than seventy percent by volume, with about eighty percent to about ninety-five percent being typical. An unbacked PDC can be mechanically bonded to a tool (not shown), according to one example. Alternatively, the PDC can be bonded to a substrate, thereby forming a PDC cutter, which is typically insertable within a downhole tool (not shown), such as a drill bit or a reamer.
FIG. 1 shows a side view of a PDC cutter 100 having a polycrystalline diamond (“PCD”) cutting table 110, or compact, in accordance with the prior art. Although a PCD cutting table 110 is described in the exemplary embodiment, other types of cutting tables, including cubic boron nitride (“CBN”) compacts, are used in alternative types of cutters. Referring to FIG. 1, the PDC cutter 100 typically includes the PCD cutting table 110 and a substrate 150 that is coupled to the PCD cutting table 110. The PCD cutting table 110 is about one hundred thousandths of an inch (2.5 millimeters) thick; however, the thickness can vary depending upon the application.
The substrate 150 includes a top surface 152, a bottom surface 154, and a substrate outer wall 156 that extends from the circumference of the top surface 152 to the circumference of the bottom surface 154. The PCD cutting table 110 includes a cutting surface 112, an opposing surface 114, and a PCD cutting table outer wall 116 that extends from the circumference of the cutting surface 112 to the circumference of the opposing surface 114. According to some exemplary embodiments, a bevel (not shown) is formed around at least the circumference of the PCD cutting table 110. The opposing surface 114 of the PCD cutting table 110 is coupled to the top surface 152 of the substrate 150. Typically, the PCD cutting table 110 is coupled to the substrate 150 using a HPHT press. However, other methods known to people having ordinary skill in the art can be used to couple the PCD cutting table 110 to the substrate 150. In one embodiment, upon coupling the PCD cutting table 110 to the substrate 150, the cutting surface 112 of the PCD cutting table 110 is substantially parallel to the bottom surface 154 of the substrate 150. Additionally, the PDC cutter 100 has been illustrated as having a right circular cylindrical shape; however, the PDC cutter 100 is shaped into other geometric or non-geometric shapes in other embodiments. In certain embodiments, the opposing surface 114 and the top surface 152 are substantially planar; however, the opposing surface 114 and the top surface 152 can be non-planar in other embodiments.
According to one example, the PDC cutter 100 is formed by independently forming the PCD cutting table 110 and the substrate 150, and thereafter bonding the PCD cutting table 110 to the substrate 150. Alternatively, the substrate 150 is initially formed and the PCD cutting table 110 is then formed on the top surface 152 of the substrate 150 by placing polycrystalline diamond powder onto the top surface 152 and subjecting the polycrystalline diamond powder and the substrate 150 to a high temperature and high pressure process. Although two methods of forming the PDC cutter 100 have been briefly mentioned, other methods known to people having ordinary skill in the art can be used.
According to one example, the PCD cutting table 110 is bonded to the substrate 150, formed from a material such as cemented tungsten carbide, by subjecting a layer of diamond powder and a mixture of tungsten carbide and cobalt powders to HPHT conditions. The cobalt diffuses into the diamond powder during processing and therefore acts as both a catalyst/solvent for the sintering of the diamond powder to form diamond-diamond bonds and as a binder for the tungsten carbide. Voids are formed between the carbon-carbon bonds of the diamond. Strong bonds are formed between the PCD cutting table 110 and the cemented tungsten carbide substrate 150. The diffusion of cobalt into the diamond powder results in cobalt being deposited within the voids formed within the PCD cutting table 110. Although some materials, such as tungsten carbide and cobalt, have been provided as examples, other materials known to people having ordinary skill in the art can be used to form the substrate 150, the PCD cutting table 110, and form bonds between the substrate 150 and the PCD cutting table 110.
Since the cobalt, or catalyst material, is deposited within the voids formed within the PCD cutting table 110 and cobalt has a much higher thermal expansion rate than diamond, the PCD cutting table 110 becomes thermally degraded at temperatures above about 750 degrees Celsius and its cutting efficiency deteriorates significantly. Hence, typical leaching processes, which are known to people having ordinary skill in the art, have been used to react the deposited catalyst material, thereby removing the catalyst material from the voids.
All typical leaching processes involve the presence of an acid solution (not shown) which reacts with the catalyst material that is deposited within the voids of the PCD cutting table 110. According to one example of a typical leaching process, the PDC cutter is placed within an acid solution (not shown) such that at least a portion of the PCD cutting table 110 is submerged within the acid solution. The acid solution reacts with the catalyst material along the outer surfaces of the PCD cutting table 110. The acid solution slowly moves inwardly within the interior of the PCD cutting table 110 and continues to react with the catalyst material. However, as the acid solution moves further inwards, the reaction byproducts become increasingly more difficult to remove; and hence, the rate of leaching slows down considerably. For this reason, a tradeoff occurs between leaching process duration, wherein costs increase as the leaching duration increases, and catalyst removal depth.
FIG. 2 shows a perspective view of a thermally stable shell 200 of the PCD table 110 of FIG. 1 in accordance with the prior art. The thermally stable shell 200 is the portion of the PCD cutting table 110 (FIG. 1) that has been leached. The thermally stable shell 200 is formed along the outer surfaces of the PCD cutting table 110 (FIG. 1) using typical leaching processes and extends a catalyst removal depth 210 from the outer surfaces. Thus, the thermally stable shell 200 includes the cutting surface 112 and the PCD cutting table outer wall 116 of the PCD cutting table 110 (FIG. 1) and extends inwardly for about the catalyst removal depth 210. The thermally stable shell 200 is substantially cup-shaped and forms a cavity 215 therein. The cavity 215 is occupied by a catalyst rich PCD cutting table 310 (FIG. 3A). Thus, the PCD cutting table 110 (FIG. 1) includes the thermally stable shell 200 and the catalyst rich PCD cutting table 310 (FIG. 3A). The typical leaching processes involve the removal of catalyst material from a portion of the PCD cutting table 110 (FIG. 1), thereby forming the thermally stable shell 200. Usually, the catalyst removal depth 210 is uniform which is dictated by the leaching process governing parameters; however, the catalyst removal depth 210 can be non-uniform in certain examples. The catalyst removal depth 210 typically ranges from about two thousandths of an inch (0.05 millimeters) to about eight thousandths of an inch (0.2 millimeters), but can be greater in certain embodiments. The thermally stable shell 200 is substantially free of catalyst material and therefore provides a much greater thermal stability allowing the PDC cutter 100 (FIG. 1) to withstand the high flash tip temperatures generated by the interaction between rock and PDC cutter 100 (FIG. 1). The lack of catalyst material within the thermally stable shell 200 avoids the damage caused at a microscopic scale by the differential in the thermal expansion between the diamond network and the catalyst material and delays the onset of the diamond graphitization process.
FIG. 3A shows a perspective view of the PCD cutting table 110 developing a wear flat 300 in accordance with the prior art. The PCD cutting table 110 includes the thermally stable shell 200 surrounding portions of the catalyst rich PCD cutting table 310. As a portion of the thermally stable shell 200 is worn out by the interaction between the PCD cutting table 110 and the rock formation, the wear flat 300 is formed, thereby exposing a portion of the catalyst rich PCD cutting table 310. Hence, the wear flat 300 produces an interface 305 between the thermally stable shell 200 and the portion of the catalyst rich PCD cutting table 310. The portion of the catalyst rich PCD cutting table 310 also begins interacting with the rock formation along with the interaction between the thermally stable shell 200 and the rock formation, thereby speeding up the thermo-mechanical wear process of the PCD cutting table 110. This leads to a dramatic loss of cutting efficiency and greatly reduces the remaining life of the PDC cutter 100 (FIG. 100). As the thermally stable shell 200 is worn out and the portion of the catalyst rich PCD cutting table 310 becomes exposed, a second failure mechanism also occurs. The second failure mechanism involves having a portion of the thermally stable shell 200 and the portion of the catalyst rich PCD cutting table 310 both interacting with the rock formation. During the drilling application, cracks are forming at the interface 305 and the contact point of the interface 305 with the rock formation. Eventually, chips are created within the PCD cutting table 110, thereby accelerating PDC cutter 100 (FIG. 1) degradation.
FIG. 3B shows a perspective view of the PCD cutting table 110 developing a larger wear flat 350 in accordance with the prior art. As the drilling application continues and more rock is removed by the shearing action of the PCD cutting table 110, the size of the wear flat 350 increases, thereby exposing a larger portion of the catalyst rich PCD cutting table 310. As the wear progresses, the rate of damage accelerates caused by the thermal effect because there is a larger portion of the catalyst rich PCD cutting table 310 interacting with the rock formation and less thermally stable shell 200 interacting with the rock formation. The cobalt within the larger portion of the catalyst rich PCD cutting table 310 thermally expands at a different rate than the expansion of the diamonds, thereby increasing the rate of damage.
The drawings illustrate only exemplary embodiments of the invention and are therefore not to be considered limiting of its scope, as the invention may admit to other equally effective embodiments.